The present invention relates to an electric power steering apparatus.
In recent years, electric power steering apparatuses (EPS) using a motor as a drive source have been widely adopted as power steering apparatuses for vehicles. Japanese Laid-Open Patent Publication No. 2002-104219 discloses an EPS having a torque sensor 23 which detects steering torque on the basis of the torsion angle of a torsion bar 22 provided in the middle of the steering transmission system (a steering shaft 21) connecting the steering wheel and the steered wheels, as shown in FIG. 9. This EPS controls the assist force provided in the steering system on the basis of the steering torque detected by the torque sensor 23. FIG. 9 is a diagram schematically showing the configuration of a sensor which is widely adopted as a torque sensor for EPS's, that is to say, a twin resolver type torque sensor for detecting the torsion angle of the torsion bar 22 by means of a pair of angle sensors 24a and 24b (resolvers) provided at both ends of the torsion bar 22.
At the time of braking of a vehicle, particularly at the time of sudden braking, and in cases where the road on which the vehicle is running is a low μ road sloping downward, the orientation of the vehicle sometimes deflects due to a lack of, for example, weight balance or braking force balance, despite the fact that the steering angle is constant, for example when the brakes are applied during straight driving. In such cases, self aligning torque (SAT) for turning the steered wheels in the direction opposite to the direction in which the vehicle deflects acts on the steered wheels. In the case where the vehicle 25 deflects to the right, as shown in FIG. 10, for example, the self aligning torque acts on the steered wheels 26 in such a direction that the steered wheels 26 turn to the left, that is to say, in such a direction that the orientation of the vehicle is stabilized.
In the cases described above, however, the steering torque in the direction in which the vehicle deflects is detected by the torque sensor on the basis of the torsion of the torsion bar which is caused when the driver maintains the steering angle, that is to say, torsion of the torsion bar which is caused when the rotation of the steering wheel generated on the basis of the self aligning torque is restricted. Therefore, in vehicles having EPS, the EPS operates on the basis of the steering torque detected by the torque sensor, and thus, assist force is provided in such a direction that the self aligning torque which acts in such a direction as to stabilize the vehicle is canceled. As a result, a problem arises, such that the vehicle drifts in the direction in which the vehicle deflects.